The invention relates to a process for the preparation of composite pigments containing a substrate having an average diameter of 3-200 .mu.m as support for small organic or inorganic pigment particles having a diameter of less than 1 .mu.m.
Conventional organic or inorganic colored pigment particles are typically absorption pigments which absorb a portion of the incident light and reflect the remaining light in a substantially diffuse manner, thus producing the complementary color of the absorbed light. Pigment particles of this type in general have a rough, irregular surface and are opaque to a greater or lesser extent. The optical properties are affected by the average size of the pigment particles, as a result of which the color purity and color strength in general increases with decreasing average particle size. Pigment particles having an average diameter of less than 1 .mu.m have particularly favorable optical properties.
On the other hand, ultrafine pigment particles of this type are distinguished by poor dispersibility which increases with decreasing particle size.
U.S. Pat. No. 4,435,220 and Cosmetics and Toiletries, 104 (1987) 57 have proposed to apply the ultrafine pigment particles to fairly large substrates, in particular platelet-like substrates. This results in a substantial improvement in the dispersibility of the small pigment particles while simultaneously maintaining their excellent optical properties. The pigment particles are applied to the substrates, for example, by a wet chemical method. To this end, an aqueous suspension of the substrates is prepared and the pigment particles are precipitated onto the substrates, and the precipitation conditions (precipitation rate, pH, etc.) are selected such that small pigment particles and no continuous layer are formed (U.S. Pat. No. 4,545,821, EP 0 246,523). However, this process is relatively complicated and requires accurate control of the deposition conditions. Furthermore, defined composite pigments which, in the first layer on the substrate particles, have a relatively high degree of coverage with pigment particles and do not grow in an uncontrolled manner towards the outside are only obtained for relatively low weight proportions of the pigment particles of typically no more than 25% by weight, relative to the weight of the pigment. A further disadvantage is that pigment particles which cannot be precipitated in water cannot be applied using this method.